Wells for the production of hydrocarbons are designed in a range of different ways, depending on many influencing factors. Such factors include production characteristics, safety, well servicing, installation- and re-completion issues, downhole monitoring and control requirements and compartmentalisation of producing zones.
Further, as wells mature, they are normally serviced using techniques known as per se on regular intervals.
Intervention services such as wireline and coil tubing are most commonly applied. The service could, as an example, be conducted for data acquisition purposes, for zone isolation or opening for production from new zones, for zone stimulation, for removal of salt deposits or to fix leakages in the wells tubular.
Common well components such as plugs and packers for isolation purposes, valves such as flow control valves or choke valves, data acquisition devices such as pressure-, temperature, flow rate and flow composition meters may be utilised in conjunction with a well, either as a part of the well completion (incorporated as part of the well's tubular) or as intervention tools (intervened in the well and in some cases left in the well, permanently or on a long term basis, attached to the well tubular using techniques as known per se).
The installation of the production tubular, including a selection of the above described components, and the wellhead is referred to as completing the well. Many of the above described devices can be installed as an integrated part of the well completion (tubular). In many cases, a selection of said devices can be remotely operated via control lines (hydraulic or electric lines). Such control lines can be hydraulic and/or electrical and/or fibre-optic lines that run all the way from the reservoir section(s) of a well to the surface.
Evolution of oil wells has entailed methods and well designs such as multi lateral wells and side-tracks and smart well completions. A multilateral well is a well with several “branches” in the form of drilled bores that origin from the main bore. The method enables a large reservoir area to be drained by means of one well. A side track well is typically an older production well that is used as the basis for drilling one/multiple new bores. Hence, only the bottom section of the new producing interval need to be drilled, hence time and costs are saved.
Smart well completions are typically applied in wells with several producing and/or injecting zones and/or wells with several bores (i.e. multilateral wells). Said smart well completions normally comprise a series of monitoring systems and/or valves incorporated as integrated parts of the production tubular, to monitor and control production from each producing interval in the well or injection into each injection interval in the well. Smart well monitoring systems and valves are normally operated remotely through hydraulic and/or electric communication (and in some cases partly fibre optic) lines that run all the way from the reservoir section(s) of a well to the surface. Often, as a backup solution, smart well valves can also be manipulated by an intervention operation (such as coiled tubing, wireline, or slickline), should the remote activation systems for some reason fail to operate. Smart well valves may comprise on/off valves (i.e. either fully open or fully shut) as well as variable opening chokes.
New well designs such as the ones described above have in a number of cases entailed a new challenge in the form of inaccessible areas of the well. In particular, this may apply for multilateral wells and sidetrack wells. It is normally deemed as non-desirable to perform interventions in the side branches of a well as the risk of getting stuck in the junction between branches and/or causing other types of damage to the well are perceived to be of too high a risk. Neither is it in most cases possible to bring control lines into branches of a well as per today. As a consequence, measurement and control tasks in branch wells are normally limited to areas where the branch enters the main bore of the well, and can normally not be executed within the branch(es) itself.
Another example of inaccessibility related to well segments is subsea wells, where the wellheads are located on the seabed. Here, interventions such as data acquisition or barrier installation jobs are scarce due to low availability and high costs associated with required drilling rigs or intervention vessels that need to be mobilised for the work.
In addition to the problem with non-accessible wells and/or areas in wells, several other factors may inflict challenges to the operation of well equipment. Such factors include debris/fill material, corrosion, scaling (salt deposits), and damage to control lines and line connectors. As an example, debris such as sand, scale (salt deposit) particles or steel fragments from drilling or perforation operations may deposit on top of intervention plugs, making it very difficult to retrieve them after usage. Scale and corrosion on a plug itself may cause similar problems.
In summary, there is a range of possible scenarios that entail non-accessibility to or non-operability of downhole tooling required for important work in wells related to oil and gas production.
To solve the said problems related to accessibility and/or operation of the above described well components new, autonomous systems and methods related to plugs, packers, valves and monitoring systems are emerging. Further, said autonomous system commonly uses wireless communication methods for communication with control systems located at the surface of the earth or at communication nodes located elsewhere in or on the wells.
Several systems are evolving that enable wireless communication in wells related to the production of hydrocarbons. One such wireless system and method is explained in detail in patent applications NO 20044338 and NO 20044339, owned by the applicant of this patent application. Further, patent application NO 20061275, also owned by the applicant of this patent application describes an alternative wireless communication technique and related applications.
A limitation with autonomous and/or wireless based downhole application is the provision of power for system operation; as all autonomous devices are dependant on local supply of power to be operated in a proper manner.
The present invention relates generally to local, downhole electrical power generation and, in a preferred embodiment describes herein, more particularly to a power generator based on flow-induced vibration principles.
Existing Methods
In order to energize downhole wireless telemetry systems and autonomous devices it is commonly accepted to utilize non-rechargeable batteries. However, such batteries entail several challenges which limit the possible use of wireless telemetry and autonomous devices:                Non-rechargeable batteries suffer from a phenomenon referred to as “self-discharge”. Self-discharge is a natural phenomenon of a chemical system, defined as the electrical capacity that is lost when the cell simply sits on the shelf. Self-discharge is caused by electrochemical processes within the cell. At a higher temperature or with advanced age, the self-discharge rate increases substantially. Typically, the rate of self-discharge doubles with every 10° C. Even at quite common well temperature surroundings, non-rechargeable batteries can suffer from self discharge as high as >0.3% per day. The higher the downhole temperature, the lower is the lifetime, typically speaking about months in a High Pressure/High Temperature (HPHT) environment. Hence, due to the subject of self-discharge, it is a challenge to make optimum usage of the energy potential that non-rechargeable batteries represent.        Non-rechargeable batteries will in many cases provide insufficient amounts of energy required for multiple and/or high power requiring operations of a downhole device such as a valve. This entails that an autonomous system powered by a non-rechargeable battery cannot be utilized in smart well arrangements.        Wireless telemetry systems and autonomous devices powered by non-rechargeable batteries are dependent on frequent intervention to replace batteries as they are depleted for energy. This will in many cases make wireless/autonomous downhole technology undesired.        
Based on challenges described above it can be concluded that wireless telemetry systems and autonomous devices are dependant on local downhole power generation for prolonged operation as well as high temperature applications. Several methods have been patented in the industry and some are developed. However, known existing systems suffer from certain drawbacks resulting in short lifetime and/or too low energy generation levels, and may not be applicable for many autonomous systems. A selection of methods can be exemplified as follows:                Intrusive Propellers/turbines. Such approaches can provide high levels of energy generation, but are vulnerable in hydrocarbon well environments due to factors such as bearing wear, particles plugging bearings, particle wear and/or cavitations of propeller blades, and as a result it is undesired to utilize such technology for long term applications with downhole autonomous systems.        Temperature—Peltier elements. Such elements generate energy based on temperature difference between two points. The technology is not applicable in a well environment as the temperature is near constant over short distances.        Nuclear generators have a good energy potential, but also a grave pollution and risk potential.        Annulus pressure pulse generators are systems where a pump located on the surface of the earth is used to impose pressure surges in the annular space between the production tubular and the casing of the well. A downhole accumulator, located in the same annulus but in the reservoir end of the completion, is compressed at high pressure peaks and expands at low pressure peaks. This movement can directly or indirectly, by means of a flowing fluid, be used to operate a downhole turbine generator. Annulus pulse generators require that the well completion is tailored for such generation, and is therefore a poor match to retrofit systems (i.e. systems that are installed by a well servicing technique subsequent to the well completion process). Further, such generators would impact on barrier requirements, and would not be applicable in high pressurised wells because the required downhole accumulator would provide a too small mechanical working window.        
Upon careful consideration, the applicant of this patent has concluded that vibration based energy generation systems are perceived to be the best option for a long-term application in a hydrocarbon well environment.
Vibration, or more precisely flow-induced vibration generators, have been investigated and patented by the industry. Patents written as far back as 1959 (U.S. Pat. No. 2,895,063) and 1971 (U.S. Pat. No. 3,663,845) describes means of generating electric power from a flowing fluid (in this case air) which causes an object designed for the purpose to vibrate, said object being connected to a energy generating device such as a magnet and coil assembly.
As a further example, the present invention has similarities to U.S. Pat. Nos. 5,839,508 and 7,199,480. However, all the above mentioned patent documents as well as other investigated publications that describe the state of such technology are perceived to have weaknesses with respect to application in a hostile and highly pressurised downhole environment as well as gaining an optimal output of electric energy.
The latter—i.e. output of electric energy in an oil well by means of a vibration assembly—has proven, through research, to be a challenging task. As an example, power output in the order of magnitude W (Watt) may be relatively hard to achieve (output in the order of mW is more likely to expect), hence it is of great importance that downhole energy generation devices are designed for as high efficiency as possible. This may not be achievable without novel, inventive design features related to vibrating based energy generation tooling as described herein.
Based on the existing knowledge from public information, such as the likes of U.S. Pat. Nos. 2,895,063 and 3,663,845, the current invention identifies novel and inventive features required for high-pressure regime, downhole operations where the output of power needs to be optimised.